a) Field of the Invention
The present invention relates to a manufacture method of a field emission element, and more particularly to a method of manufacturing a field emission element emitting electrons from a sharp tip of an emitter.
b) Description of the Related Art
A field emission element emits electrons from a sharp tip of an emitter by utilizing electric field concentration. For example, a flat panel display can be structured by using a field emitter array (FEA) having a number of emitters disposed in array. Each emitter controls the luminance of a corresponding pixel of the display.
In a field emission element, a gate electrode is configured near an emitter electrode. By applying positive potential to the gate electrode, the emitter electrode can emit electrons to an anode electrode.
Providing that an emitting current is same or increases, the field emission element is desired to have such properties as a decreased threshold voltage between the gate electrode and emitting electrode, fast drive and reduced power consumption. Consideration in choosing a structure and shape of the element is required to obtain those properties. In addition, for a steady and easy manufacture of such element, special consideration is necessary for the manufacturing process.
The shape of the emitter electrode especially influences on the above-listed properties. Most importantly, the emitter electrode should have a fine tip. By making an apex angle of the emitter electrode as small as possible, electric field at the tip will be strong enough to reduce the threshold voltage between the gate electrode and emitting electrode. Therefore, provided that the emitting current obtained is the same, it is possible to reduce the threshold voltage. Provided that the threshold voltage is the same, it is possible to obtain a larger emitting current by the same threshold voltage between the gate electrode and emitting electrode than using the emitter electrode with a larger apex angled tip. Therefore, it is important to utilize a manufacturing method that easily enables sharpening the tip of the emitting electrode.
Especially, as shown in FIGS. 10A and 10B, a composite-shaped emitter electrode, which has an outline composed of two parts of lines (straight lines as shown in FIG. 10A or curves as shown in FIG. 10B), has a fine tip and good electrical properties. At meantime, the emitter electrode has a wide base, and so the casting of material for electrode to an emitter mold can be easily performed.
Some manufacturing methods of the composite-shaped emitter electrode have been provided conventionally.
For example, the assignee of this application discloses a manufacturing method of the emitter electrode in JP-A HEI 09-274846. The method includes steps of forming an overhung gate electrode, depositing a sacrificial film to an overhanging portion of the gate electrode by a deposition method having good step coverage, performing a wet oxidation process to the sacrificial film for volume expansion after diffusing impurities to a portion corresponding the tip of the emitter electrode. In the same processing time, a region with diffused impurities expands more likely than a region without diffused impurities. Therefore, the wet oxidation process of the sacrificial film will form an emitter electrode mold that has the composite-shaped cross-section including two different degrees of volume expansion.
Moreover, JP-A HEI 09-17335 discloses another manufacturing method of the composite-shaped emitter electrode. In the method, an emitter electrode is formed by following steps. At first, on a substrate having concaves is formed a gate oxide film having smaller openings than the concaves on the substrate. Then, an electrode material is deposited on the gate oxide film by sputtering to form gate electrodes on the gate oxide film and emitter electrodes on the concaves under the openings of the gate oxide film.
In the prior art, it is difficult to control over the oxidation process and film thickness of the gate film. Moreover, it is impossible to make an insulating film thick, and therefore, a distance between emitter gates will be too short causing low insulation-resistant, and fast drive is limited due to high capacitance.